Hawking radiation glimpsed in artificial black hole

Do-it-yourself event horizon

You might expect black holes to be, well, black, but several decades ago Stephen Hawking calculated that they should emit light. Now, for the first time, physicists claim that they have observed this weird glow in the lab.

Others are not yet convinced of the team’s evidence, or argue that Hawking radiation cannot come from anything other than a real black hole. If further experiments confirm that the new measurements, made at Insubria University in Como, Italy, are a form of Hawking radiation, however, it could open a new window into some of the most exotic objects in the universe. The finding also suggests that the bizarre physics once thought unique to black holes is much more widespread.

A black hole is an incredibly dense concentration of mass with an extreme gravitational field around it. Black holes earned their moniker because inside a certain radius, known as the event horizon, nothing escapes – not even light.

Uncertainty principle

Or so it seemed. Then in 1974 Hawking showed that, according to quantum theory, black holes should emit radiation after all. This is a consequence of the uncertainty principle, which says we can never be sure that an apparent vacuum is truly empty and, instead, that virtual particles are constantly appearing in pairs. These couples, made of a particle and its antimatter counterpart, rapidly annihilate and vanish again, so normally go unnoticed.

However, if a pair of photons pops up right at the event horizon, they may find themselves on different sides, with one flying free outside and the other trapped forever within. This prevents them from merging and vanishing, so the outside photon is effectively emitted by the black hole (see diagram, above right).

Hawking predicted black holes should give off a steady stream of such radiation – and many scientists assume he is right. The problem is that no one has ever actually seen it.

Escape velocity

In recent years, physicists have been toying with laboratory experiments that imitate the physics of an event horizon. This marks the point where escape from a black hole is impossible because the velocity required exceeds the speed of light, the cosmic speed limit.

Analogue black holes have a similar point that cannot be crossed because the speed required is too great. Unlike in a real black hole however, this “horizon” is not created by intense gravity, since we do not know how to synthesise a black hole, but by some other mechanism – utilising sound or light waves, for example. However, no one had seen photons resembling Hawking radiation emerging from these analogues, until now.

To create their lab-scale event horizon, Daniele Faccio of Heriot-Watt University in Edinburgh, UK, Francesco Belgiorno of the University of Milan, Italy, and their colleagues focused ultrashort pulses of infrared laser light at a wavelength of 1055 nanometres into a piece of glass. The extremely high intensity of these pulses – trillions of times that of sunlight – temporarily skews the properties of the glass. In particular, it boosts the glass’s refractive index, the extent to which the glass slows down light travelling through it.

The result is a moving point of very high refractive index, equivalent to a physical hill, which acts as a horizon. Photons entering the glass behind this “hill”, including ones that are part of a virtual pair, slow as they climb the hill and are unable to pass through it. Relative to the slow-moving pulse, they have come to a stop and remain behind the pulse until it has passed through the glass’s length.

Mysterious photons

To see if this lab-made event horizon was producing any Hawking radiation, the researchers placed a light detector next to the glass, perpendicular to the laser beam to avoid being swamped by its light. Some of the photons they detected were due to the infrared laser interacting with defects in the glass&colon; this generates light at known wavelengths, for example between 600 and 700 nanometres.

But mysterious, “extra” photons also showed up at wavelengths of between 850 and 900 nanometres in some runs, and around 300 nanometres in others, depending on the exact amount of energy that the laser pulse was carrying. Because this relationship between the wavelength observed and pulse energy fits nicely with theoretical calculations based on separating pairs of virtual photons, Faccio’s team concludes that the extra photons must be Hawking radiation (Physical Review Letters, in press).

Not everyone is ready to agree. Adam Helfer at the University of Missouri in Columbia says the term Hawking radiation is best reserved for actual black holes with gravitational fields. “There is a parallel between them to a certain extent, [but] the laboratory experiments, interesting as they are, do not really bear on the very deep problems which are special to black holes.” These revolve around how to fully marry gravity and quantum mechanics when describing these objects.

Quantum entanglement

In future, Ted Jacobson at the University of Maryland in College Park suggests testing for a key characteristic of Hawking radiation – whether the pairs of photons separated by the horizon are quantum entangled. Faccio says that using an optical fibre, as Leonhardt and colleagues did in 2008, rather than a glass block, might allow photon pairs separated by a laser horizon to be analysed for entanglement.

Meanwhile, Hawking radiation is also popping up in other, less direct black hole imitators. A team led by Silke Weinfurtner at the University of British Columbia in Vancouver, Canada, announced in August that they had observed a water-wave version of Hawking radiation in an experiment involving waves slowed to a halt to form a horizon (arxiv.org/abs/1008.1911).

Hawking radiation is turning out to be “a general phenomenon that occurs whenever you have a horizon of any sort”, says Matt Visser of Victoria University of Wellington, New Zealand, who was not involved in either of the experiments.

He is not the only one intrigued by the indication that the sequence of events leading to Hawking radiation arises in analogue horizons, as well as real black holes. “The line of reasoning is more generic than it might at first seem, giving more faith that it may also be right for black holes,” says Bill Unruh, who collaborated with Weinfurtner on the water-wave experiment and is known for the “Unruh effect”, a predicted phenomenon that is similar to Hawking radiation but occurs outside a black hole.

“All the pieces of the puzzle seem to have suddenly fallen together at the same time,” says Faccio. “This is all very exciting.”

This story was updated to reflect the fact that the research was led by Daniele Faccio of Heriot-Watt University in Edinburgh, UK, and that the measurements were carried out at Insubria University in Como, Italy.